What is Cleistogamy?
Argyrolobium and Lotononis both produce cleistogamous flowers, but they are far more extreme in the Silverpods.
But what is this, and why is it important?
Cleistogamy is when plants produced flowers that dont need to be pollinated. They pollinate themselves. As they dont need to attract pollinators, they are smaller and less conspicuous. In this group they are an insurance against lack of pollinators during the flowering season, and they are usually produced before and after the showy (chasmogamous) flowers that pollinators and observers notice. Typically, they are not included in flowering times for these species. And most observers dont even notice them.
Cleistogamous flowers of Argyrolobium have lost several organs (these are aborted early on during flower development).
- the standard, although reduced, is always present.
- the wings are reduced to fragile spoonlike appendages
- the keels are reduced to filaments.
- the anthers are reduced to 2 functional anthers remaining (these are the large outer abaxial anthers - the other 3 are reduced to filaments. The 5 inner filaments and anthers are totally lost). These only produce very little pollen.
- the styles are short and curve sharply inward, touching the viable anthers.
- pollen germinates in the anthers, grows through the walls and into the stigma, pollinating the ovules.
The cryptic nature of cleistogamous flowers reduces their vulnerability to pollen thieves and fruit predators. And the early and late seed production seems to escape parasitism by larva of Bruchid Beetles.
Their size also allows them to be produced when resources are low, such as for very young plants, and on the shoulders of the peak flowering season. It has been argued that when conditions are adverse and flowering is not possible, this selfing in young plants allows them to be "annuals", rather than as typical to reach peak flowering after a few years growth, and thus survive by producing seeds to another year.
Cleistogamy is a seasonal back-up to main flowering, and no species have given up normal flowers for a selfing mode of reproduction.
In short: the advantages for dimorphic species includes (1) the maximisation of seed set even when pollinators are absent, (2) reduced pollen and seed predation, (3) a longer flowering season, and (4) a precocial flowering (under occasional adverse conditions). The disadvantage of reduced gene flow is countered by an inability of chasmogamous flowers to seed seed from their own pollen.
Cleistogamy is absent in Fynbos species. Few Silverpods occur in Namaqualand, because none are annuals. However, cleistogamy does occur though throughout the range of Argyrolobium , being most prominent in the east.
Argyrolobium species with strongly dimorphic (two types: the flowering and selfing forms) flowers in South Africa:
A. ascendens, A. argenteum, A. barbatum, A. campicola, A. candicans, A. collinum, A. filiforme, A. harveyanum, A. humile, A. lotoides, A. marginatum, A. molle, A. pachyphyllum, A. pauciflorum, A. pseudotuberosum, A. pumilum, A. rotundifolium, A. rupestre, A. sericosemium, A. stipulaceum, A. tomentosum, A. tuberosum, A. velutinum.
When visiting the veld, look out for these species - they may well be in flower under your noses, albeit by inconspicuously self-indulging.
